Magnetic induction accelerator



Jan. 5, 1954 K. GUND 2,665,392

MAGNETIC INDUCTION ACCELERATOR Filed Oct. 28, 1950 Patented Jan. 5, 1954 UNITED STATES PATENT OFFICE 6; Claims;

This invention relates to a magnetic induction accelerator for electrically charged elementary particles, e. g., electrons, and is particularly con-- cerned. with an accelerator for therapeutic irradiation.

The term magnetic induction accelerator refers to a device in which elementary particles, e. g., electrons, are moved. along approximately circular paths in a magnetic. guiding field and.

accelerated by means of a magnetic flux cf increasing, strength passing through said circular paths.

One. of the principalobjects of the invention is to reduce the weight of the magnetic iron in an accelerator of the above-noted type so as to obtain appreciable reduction of its total weight and bulk for the purpose of adapting it for use as a radiation source in therapeutic irradiation apparatus.

In accordance with the invention, the guiding field poles are so dimensioned that the guiding field; strength increases, as usual, in the neighborhood of the equilibrium orbit up to the final acceleration of the electrons in proportion with the accelerating field while magnetic saturation occurs therein at an intensity or strength of the acceleration which is relatively small compared with its final intensity.

The invention is based on recognition of the fact that the electrons fill the evacuated acceleration tubeor chamber in the form of a broad band only at the commencement of acceleration, and concentrate quickly toward the equilibrium orbit, oscillating finally about said orbit within a radially narrow zone. The guiding field therefore has to be as wide as the evacuated tube only at the start of acceleration, so as to secure all electrons in their orbital equilibrium paths. The width of the guiding field may diminish with the narrowing of the electron bundle. In other words only in a portion of the guiding field which is present at the start of acceleration need the field strength increase corresponding to the 1 2 condition up to the final acceleration along the equilibrium orbit. The magnetic iron of the guiding field poles outside of the region near the equilibrium orbit and also the yoke may therefore be generally of smaller Weight and dimensions than heretofore.

In an accelerator having common excitation for the accelerating and the guiding fields, a contraction of the electron bundle to the neighborhood of the equilibrium path takes place, as taught by experience, at a time when the total excitationof the. accelerator reaches a low multiple of the value which ispresent. at the injection of the electrons. The magnetic saturation within the border zones ofthe guiding field poles therefore shouldbegin atleast atfthe tenfold value of the common. excitation which is present upon. injection. of the electrons.

During experimenting itw as foundsuitable to. permit. saturation. of the guiding field poles to about one-half of their width. at the oppositelypositioned. polesurfaces. In thiscase the-saturation of the circular poles only at the outer marginal. zone of the pole surfaces is preferable to a saturation including. the inner marginal zone;

because a greater flux is saved. by narrowing the guiding field width outside of the equilibrium:

orbit, on accountoi the greater surface permeated" by the flux, than by corresponding diminution of'the width of the guiding field within the equilibrium circuit;

The arrangement according to the invention:

made it possible, e. g... in the case of, an accelerator fer a final energy of' 12 m.. e. v., in additionto a broadening. ofthe. guiding field. by 30%,-to reduce the magnetic flux by 15% (as compared with an. otherwise similar priorstr-ucture): and

to reduce thereby the. cross-sectional areaoi' the:

yoke of the electromagnet, which. is; common. tothe accelerating. and. to the guiding field, to suchan extent as to. result in. reduction of the weight of the structure by about 10 The objects and features noted in the foregoing outline and additional objects and features will appear from the following description of the straying field near the edge of the air gap. The toroidal accelerating tube or chamber 9 is disposed between the opposite guiding field poles 5, 6 and provides therein a closed orbital path along which the electrons are accelerated. The

guiding field poles 5, 6 form an inner guiding field zone which embraces the equilibrium paths of the electron particles with-in the tube 9 and are provided with generally ring-shaped magnetic pole shoes 10, H which are integral therewith and extend generally radially outwardly therefrom, forming an outer guiding field zone. The pole shoes 10, I I are about as wide as the guiding field poles 5, and somewhat wider than the tube 9. These shoes are so dimensioned that their saturation begins at the most at such time when the excitation caused by the coils l2, 13 has reached about tenfold value of the excitation at the injection of the electrons. From this moment on, the guiding field flux increases in accordance with the 1:2 condition only in the poles 5, 6 and not any more in the shoes extending therefrom. In other words, the field strength in said inner guiding field zone controlled by the poles 5, 6 increases substantially proportional with the field strength of the acceleration field, while the ring-shaped pole shoes 10, H become magnetically saturated at a field strength of the acceleration field which is small as compared with the final field strength thereof to cause further increase of the field strength in said outer guiding field zone which is less than proportional with the further increase of the field strength in said acceleration field. Accordingly, the total flux in the yoke does not increase to an extent as would be the case if the 1 2 condition were maintained for the entire guiding field.

Certain structural and functional features of the accelerator, which are not shown in the drawing or explained in the foregoing specification may be had by consulting copending applications Ser. Nos. 192,731, 192,732 and 226,729, filed October 28, 1950, and features disclosed in these copending applications which are deemed pertinent to the present invention should be treated as if they had been included herein.

The appended claims define what is believed to 'be new and desired to have protected by Letters Patent of the United States.

I claim:

1. A magnetic induction accelerator for electrically charged particles comprising an annular tube providing therein a closed orbital path along which the particles are accelerated, a magnetic yoke, accelerating pole pieces magnetically cou pled to said yoke forming a path for the magnetic flux of the acceleration field which extends centrally of said tube, magnetic guiding field poles projecting generally radially outwardly from said acceleration pole pieces forming outside of said tube on opposite sides thereof annular opposing inner surfaces defining an inner guiding field zone which embraces the equilibrium paths of said particles within said tube, and generally annularly extending magnetic pole shoes projecting generally radially outwardly from said guiding field poles forming outside of said tube on opposite sides thereof annular axially opposing inner surfaces defining an outer guiding field zone.

2. The structure defined in claim 1, wherein the radial width of said ring-shaped magnetic pole shoes defining said outer guiding field zone corresponds approximately to the radial width of the guiding field poles which define said inner guiding field zone.

3. The structure defined in claim 1, wherein said acceleration pole pieces and the guiding field poles and said ring-shaped pole shoes are formed by common integral members.

4. The structure defined in claim 1, wherein said acceleration pole pieces and the guiding field poles and said ring-shaped pole shoes are formed by common integral members, each member having slots to separate the accelerating pole piece from the guiding field poles thereof.

5. The structure defined in claim 1, wherein said accelerating pole pieces form inner pole surfaces separated by an axial gap along which extends said path for the magnetic fiux of the acceleration field.

6. The structure defined in claim 1, wherein the mass of the magnetically effective pole shoes defining said outer guiding field zone is small as compared with the mass of the field poles defining said inner guiding field zone.

KONRAD GUND.

References Cited in the file of this patent UNITED STATES PATENTS 

